This study will be a prospective, unblinded, non-randomized, open-label, multi-center Phase II/III study with 2 segments, i.e. a PK evaluation (Segment I), and an assessment of prophylaxis in surgical interventions and pregnancy/delivery, (Segment II). During the PK segment, the subjects would remain on their current anticoagulation therapy except for subjects on heparin therapy where a wash-out period of at least 5 half lives would be required. In total, 15 subjects with congenital ATIII Deficiency will be enrolled for the PK assessment (Segment I).